Coupling and Cohesion, Coupling, Cohesion

Coupling refers to the degree to which one class knows about or uses members of another class.
Loose coupling is the desirable state of having classes that are well encapsulated, minimize references to each other, and limit the breadth of API usage.
Tight coupling is the undesirable state of having classes that break the rules of loose coupling.
Cohesion refers to the degree in which a class has a single, well-defined role or responsibility.
High cohesion is the desirable state of a class whose members support a single, well-focused role or responsibility.
Low cohesion is the undesirable state of a class whose members support multiple, unfocused roles or responsibilities.